Thermocouples are a widely used type of temperature sensor. They can measure a wide range of temperatures, they are very simple in operation and measure the temperature between two points.
A thermocouple construction consists of two dissimilar metal wire welded together at the measuring point and insulated from each other. It will usually have an outer protection sheath.
If two conductors of different materials are joined at one point, an EMF (electromotrice force) is created between the open ends which is dependent upon the temperature of the junction. In most applications, one of the junctions — the "cold junction" — is maintained at a known (reference) temperature, while the other end is attached to a probe. Another temperature sensor will measure the temperature at this point, so that the temperature at the probe tip can be calculated.
Usually the thermocouple is attached to the indicating device by a special wire known as the "compensating" or "extension" cable. This is manufactured to have approximately the same temperature characteristic as the thermocouple, and so generate a voltage proportional to the difference between the hot junction and cold junction. The cable is connected so that the additional voltage is added to the thermocouple voltage, thus compensating for the temperature difference between the hot and cold junctions.
A variety of thermocouples is available, suitable for different measuring applications.